The present applicant proposed a manufacturing method of an optical element such as a micro lens, etc. in an earlier application (JP-A-2004-271756, which corresponds to US 2004-0173862-A1). In this manufacturing method, a silicon substrate is trench-etched and many trenches are arranged in parallel. A silicon oxide is buried within each trench by thermal oxidation. The micro lens integrated with the silicon substrate is made by replacing a silicon layer between the respective trenches with the silicon oxide.
The following contents have been found by subsequent consideration. Each trench is formed by using an etching mask patterned on the basis of a resist mask and processing the silicon substrate by D-RIE (Deep Reactive Ion Etching). The width of the trench is dispersed by dispersions respectively caused in resist mask making, etching mask making and trench processing. Therefore, it has been found that there is a fear that no trench interior can be buried by the silicon oxide by the thermal oxidation without any clearance.
For example, when the resist mask is made by semiconductor photolithography, a dispersion of about 0.2 μm is generated with respect to a groove width of 2 μm. Further, when the etching mask is made, the width of the groove corresponding to the trench is dispersed by, e.g., 0.1 to 0.2 μm. Further, when the trench is processed by using the etching mask, the width of the trench is dispersed by, e.g., 0.1 to 0.2 μm.
When a clearance is left within the trench by such dispersions at the processing time, a diffraction phenomenon is generated and diffraction light is advanced in a direction different from an original converging and refracting direction. Therefore, there is a fear that efficiency is reduced.
In a method using a liquid phase represented by the sol-gel method, it is difficult to invade a liquid into the trench of a high aspect ratio since viscosity of the liquid is high. Further, in a gaseous phase method represented by CVD, the trench can be buried, but a film forming speed must be greatly reduced. Further, in a physical film forming method represented by sputtering and evaporation, a sidewall within the trench becomes shade. Therefore, it is difficult to bury the trench completely.
In particular, when each of the above methods is applied to the trench of a high aspect ratio larger than ten, a problem exists in that an opening portion of the trench is early narrowed and a clearance is left within the trench. FIG. 27 is a schematic view showing a state in which the clearance is left within the trench. A silicon oxide film 32 is formed on an inner wall face of the trench 31 formed in the silicon substrate 30. However, the opening portion is narrowed by the silicon oxide film 32, and it becomes difficult to supply the silicon oxide into the trench 31, and the clearance 33 is left within the trench 31.
As mentioned above, it has been found that there is a fear that the clearance is left within the trench.